1. Field of the Invention
This invention belongs to the field of fermentation processes and, more particularly, to the fermentative conversion of sugars derived from such sources as grains, tubers, starch legumes, sugar cane, agricultural wastes, municipal wastes, wood, sawdust, bark, and the like, to ethanol.
2. Description of the Prior Art
In recent years, considerable attention has been given to the conversion of biomass to liquid fuels and chemicals. Biomass offers the potential to replace or supplement dwindling reserves of non-renewable fossil fuels with fuels derived from vegetative, and therefore, renewable, carbon-containing sources such as amylaceous grains and tubers, sugar cane, wood and other cellulosic sources including cellulose-containing municipal wastes (newsprint, cardboard, etc.), and similar materials. The carbohydrate contained in the foregoing materials is first hydrolyzed to fermentable sugar such as glucose (dextrose), fructose, maltose and sucrose, and the fermentable sugar is thereafter converted by fermentation to ethanol. Hydrolysis and fermentation can be carried out in individual vessels or side-by-side in a single vessel. It is well known that the maximum concentration of ethanol which can be achieved by fermentation is limited by the tolerance of the ethanol-producing microorganisms, e.g., brewers' yeast, Clostridium sp., etc., for ethanol such that as the concentration of the ethanol increases, the ability of the microorganisms to convert further quantities of fermentable sugar to ethanol decreases. At ethanol concentrations above 2% by weight of the fermentation medium, the rate of fermentation begins to decline noticeably with the fall-off in production being particularly apparent when the 5% level has been exceeded. At ethanol concentrations of about 10-12%, fermentation ceases and so this level of ethanol represents an inherent limitation on the productivity of fermentation processes.
In order to achieve maximum production of ethanol from a given volume of fermentation equipment, it is desirable to limit the concentration of ethanol in the fermenter to no higher than about 5%, and preferably, no higher than about 2%. With current fermentation procedures, this has been impractical since it requires processing unacceptably large quantities of liquid. It has been proposed to operate fermenters under vacuum so that the ethanol will volatilize as soon as it is produced thereby making it possible to maintain a low concentration of ethanol in an on-going fermentation. However, vacuum fermentation equipment is expensive to fabricate, use and maintain and its commercial practicality has therefore yet to be established.